Ceramide, which is one of sphingolipids, is a lipid that presents in an entire living body in a minute amount, but accounts for more than half of the lipids in the horny layer, which is an outermost layer of the skin, and the ceramide plays an important role in the moisturizing mechanism and barrier mechanism of the skin. The ceramide is known to exert its moisturizing function through the formation of a lamella structure after being produced in epidermal cells and secreted in between corneocytes.
However, many reports indicate that, in the case of a skin disease such as dry skin, rough skin, atopic dermatitis, senile xerosis, or psoriasis, healthy metabolism of ceramide is inhibited, to thereby lower the amount of ceramide in lipids between corneocytes, resulting in, for example, deteriorating the moisturizing function or barrier function of the skin.
For such skin diseases, attempts have been made as methods of supplementing the decreased ceramide from outside, but there is such a problem that along-term effect cannot be obtained.
Further, in recent years, it has been reported that phenomena such as apoptosis, differentiation induction, and proliferation inhibition are induced by enhancing production of intercellular ceramide, and thus the ceramide has attracted attention as an intercellular signal molecule for controlling cell proliferation, differentiation, apoptosis or the like. For example, it is known that addition of C2-ceramide (D-erythro-N-acetylsphingosine), which is a ceramide analogue, from the outside of cells induces phenomena such as apoptosis, differentiation induction, and proliferation suppression, or a treatment of cells with bacterial sphingomyelinase causes accumulation of ceramide due to degradation of sphingomyelin to thereby induce inhibition of cell proliferation and apoptosis as in the case of addition of C2-ceramide.
Therefore, it is considered that a substance which promotes production of ceramide is expected to have effects such as proliferation inhibition, differentiation induction, and apoptosis induction of animal cells, and furthermore is expected to have therapeutic effects on diseases due to cell proliferation or abnormal differentiation, such as inflammatory diseases and malignant tumors (Non Patent Document 1).
Moreover, it has been reported that the ceramide has an effect of inhibiting bone resorption, an effect of strengthening bone, and an effect of inhibiting decrease in alveolar bone, and is useful for preventing and ameliorating bone and joint diseases such as osteoporosis, bone fracture, low back pain, and rheumatism (Patent Document 1), and has an effect of preventing periodontal diseases (Patent Document 2). Therefore, a ceramide production-promoting substance is expected to exert therapeutic effects on such diseases.
Moreover, it has been reported that the ceramide has an effect of imparting firmness or hardness to the hair and an effect of improving the feel of the hair (Patent Document 3), and the ceramide production-promoting substance may be expected to exert such effects.
On the other hand, 1,3-dialkylglyceryl 2-phosphate, which is phosphorylated glyceryl ether, or a salt thereof is known to have a skin-washing effect (Patent Document 4). However, it has not been known that 1,3-dialkylglyceryl 2-phosphate or a salt thereof have a ceramide production-promoting effect and a moisturizing effect.
Moreover, a phosphorylated glyceryl ether derivative such as a salt of 1-alkylglycerol-3-phosphate is known to have an effect of improving elasticity of the skin and an effect of tightening up the skin (Patent Document 5) and has been reported to be used as a bath agent (Patent Document 6), etc. However, it has been completely unknown that the phosphorylated glyceryl ether derivative has an effect of promoting production of ceramide.